1 /*
2 * Copyright (c) 1997, 2012, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation.
8 *
9 * This code is distributed in the hope that it will be useful, but WITHOUT
10 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
11 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
12 * version 2 for more details (a copy is included in the LICENSE file that
13 * accompanied this code).
14 *
15 * You should have received a copy of the GNU General Public License version
16 * 2 along with this work; if not, write to the Free Software Foundation,
17 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
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23 */
24
25 #ifndef SHARE_VM_MEMORY_GCLOCKER_HPP
26 #define SHARE_VM_MEMORY_GCLOCKER_HPP
27
28 #include "gc_interface/collectedHeap.hpp"
29 #include "memory/genCollectedHeap.hpp"
30 #include "memory/universe.hpp"
31 #include "oops/oop.hpp"
32 #ifdef TARGET_OS_FAMILY_linux
33 # include "os_linux.inline.hpp"
34 # include "thread_linux.inline.hpp"
35 #endif
36 #ifdef TARGET_OS_FAMILY_solaris
37 # include "os_solaris.inline.hpp"
38 # include "thread_solaris.inline.hpp"
39 #endif
40 #ifdef TARGET_OS_FAMILY_windows
41 # include "os_windows.inline.hpp"
42 # include "thread_windows.inline.hpp"
43 #endif
44 #ifdef TARGET_OS_FAMILY_bsd
45 # include "os_bsd.inline.hpp"
46 # include "thread_bsd.inline.hpp"
47 #endif
48
49 // The direct lock/unlock calls do not force a collection if an unlock
50 // decrements the count to zero. Avoid calling these if at all possible.
51
52 class GC_locker: public AllStatic {
53 private:
54 // The _jni_lock_count keeps track of the number of threads that are
55 // currently in a critical region. It's only kept up to date when
56 // _needs_gc is true. The current value is computed during
57 // safepointing and decremented during the slow path of GC_locker
58 // unlocking.
59 static volatile jint _jni_lock_count; // number of jni active instances.
60
61 static volatile jint _lock_count; // number of other active instances
62 static volatile bool _needs_gc; // heap is filling, we need a GC
63 // note: bool is typedef'd as jint
64 static volatile bool _doing_gc; // unlock_critical() is doing a GC
65
66 static jlong _wait_begin; // Timestamp for the setting of _needs_gc.
67 // Used only by printing code.
68
69 #ifdef ASSERT
70 // This lock count is updated for all operations and is used to
71 // validate the jni_lock_count that is computed during safepoints.
72 static volatile jint _debug_jni_lock_count;
73 #endif
74
75 // Accessors
76 static bool is_jni_active() {
77 assert(_needs_gc, "only valid when _needs_gc is set");
78 return _jni_lock_count > 0;
79 }
80
81 // At a safepoint, visit all threads and count the number of active
82 // critical sections. This is used to ensure that all active
83 // critical sections are exited before a new one is started.
84 static void verify_critical_count() NOT_DEBUG_RETURN;
85
86 static void jni_lock(JavaThread* thread);
87 static void jni_unlock(JavaThread* thread);
88
89 static bool is_active_internal() {
90 verify_critical_count();
91 return _lock_count > 0 || _jni_lock_count > 0;
92 }
93
94 public:
95 // Accessors
96 static bool is_active() {
97 assert(_needs_gc || SafepointSynchronize::is_at_safepoint(), "only read at safepoint");
98 return is_active_internal();
99 }
100 static bool needs_gc() { return _needs_gc; }
101
102 // Shorthand
103 static bool is_active_and_needs_gc() {
104 // Use is_active_internal since _needs_gc can change from true to
105 // false outside of a safepoint, triggering the assert in
106 // is_active.
107 return needs_gc() && is_active_internal();
108 }
109
110 // In debug mode track the locking state at all times
111 static void increment_debug_jni_lock_count() {
112 #ifdef ASSERT
113 assert(_debug_jni_lock_count >= 0, "bad value");
114 Atomic::inc(&_debug_jni_lock_count);
115 #endif
116 }
117 static void decrement_debug_jni_lock_count() {
118 #ifdef ASSERT
119 assert(_debug_jni_lock_count > 0, "bad value");
120 Atomic::dec(&_debug_jni_lock_count);
121 #endif
122 }
123
124 // Set the current lock count
125 static void set_jni_lock_count(int count) {
126 _jni_lock_count = count;
127 verify_critical_count();
128 }
129
130 // Sets _needs_gc if is_active() is true. Returns is_active().
131 static bool check_active_before_gc();
132
133 // Stalls the caller (who should not be in a jni critical section)
134 // until needs_gc() clears. Note however that needs_gc() may be
135 // set at a subsequent safepoint and/or cleared under the
136 // JNICritical_lock, so the caller may not safely assert upon
137 // return from this method that "!needs_gc()" since that is
138 // not a stable predicate.
139 static void stall_until_clear();
140
141 // Non-structured GC locking: currently needed for JNI. Use with care!
142 static void lock();
143 static void unlock();
144
145 // The following two methods are used for JNI critical regions.
146 // If we find that we failed to perform a GC because the GC_locker
147 // was active, arrange for one as soon as possible by allowing
148 // all threads in critical regions to complete, but not allowing
149 // other critical regions to be entered. The reasons for that are:
150 // 1) a GC request won't be starved by overlapping JNI critical
151 // region activities, which can cause unnecessary OutOfMemory errors.
152 // 2) even if allocation requests can still be satisfied before GC locker
153 // becomes inactive, for example, in tenured generation possibly with
154 // heap expansion, those allocations can trigger lots of safepointing
155 // attempts (ineffective GC attempts) and require Heap_lock which
156 // slow down allocations tremendously.
157 //
158 // Note that critical regions can be nested in a single thread, so
159 // we must allow threads already in critical regions to continue.
160 //
161 // JNI critical regions are the only participants in this scheme
162 // because they are, by spec, well bounded while in a critical region.
163 //
164 // Each of the following two method is split into a fast path and a
165 // slow path. JNICritical_lock is only grabbed in the slow path.
166 // _needs_gc is initially false and every java thread will go
167 // through the fast path, which simply increments or decrements the
168 // current thread's critical count. When GC happens at a safepoint,
169 // GC_locker::is_active() is checked. Since there is no safepoint in
170 // the fast path of lock_critical() and unlock_critical(), there is
171 // no race condition between the fast path and GC. After _needs_gc
172 // is set at a safepoint, every thread will go through the slow path
173 // after the safepoint. Since after a safepoint, each of the
174 // following two methods is either entered from the method entry and
175 // falls into the slow path, or is resumed from the safepoints in
176 // the method, which only exist in the slow path. So when _needs_gc
177 // is set, the slow path is always taken, till _needs_gc is cleared.
178 static void lock_critical(JavaThread* thread);
179 static void unlock_critical(JavaThread* thread);
180
181 static address needs_gc_address() { return (address) &_needs_gc; }
182 };
183
184
185 // A No_GC_Verifier object can be placed in methods where one assumes that
186 // no garbage collection will occur. The destructor will verify this property
187 // unless the constructor is called with argument false (not verifygc).
188 //
189 // The check will only be done in debug mode and if verifygc true.
190
191 class No_GC_Verifier: public StackObj {
192 friend class Pause_No_GC_Verifier;
193
194 protected:
195 bool _verifygc;
196 unsigned int _old_invocations;
197
198 public:
199 #ifdef ASSERT
200 No_GC_Verifier(bool verifygc = true);
201 ~No_GC_Verifier();
202 #else
203 No_GC_Verifier(bool verifygc = true) {}
204 ~No_GC_Verifier() {}
205 #endif
206 };
207
208 // A Pause_No_GC_Verifier is used to temporarily pause the behavior
209 // of a No_GC_Verifier object. If we are not in debug mode or if the
210 // No_GC_Verifier object has a _verifygc value of false, then there
211 // is nothing to do.
212
213 class Pause_No_GC_Verifier: public StackObj {
214 private:
215 No_GC_Verifier * _ngcv;
216
217 public:
218 #ifdef ASSERT
219 Pause_No_GC_Verifier(No_GC_Verifier * ngcv);
220 ~Pause_No_GC_Verifier();
221 #else
222 Pause_No_GC_Verifier(No_GC_Verifier * ngcv) {}
223 ~Pause_No_GC_Verifier() {}
224 #endif
225 };
226
227
228 // A No_Safepoint_Verifier object will throw an assertion failure if
229 // the current thread passes a possible safepoint while this object is
230 // instantiated. A safepoint, will either be: an oop allocation, blocking
231 // on a Mutex or JavaLock, or executing a VM operation.
232 //
233 // If StrictSafepointChecks is turned off, it degrades into a No_GC_Verifier
234 //
235 class No_Safepoint_Verifier : public No_GC_Verifier {
236 friend class Pause_No_Safepoint_Verifier;
237
238 private:
239 bool _activated;
240 Thread *_thread;
241 public:
242 #ifdef ASSERT
243 No_Safepoint_Verifier(bool activated = true, bool verifygc = true ) :
244 No_GC_Verifier(verifygc),
245 _activated(activated) {
246 _thread = Thread::current();
247 if (_activated) {
248 _thread->_allow_allocation_count++;
249 _thread->_allow_safepoint_count++;
250 }
251 }
252
253 ~No_Safepoint_Verifier() {
254 if (_activated) {
255 _thread->_allow_allocation_count--;
256 _thread->_allow_safepoint_count--;
257 }
258 }
259 #else
260 No_Safepoint_Verifier(bool activated = true, bool verifygc = true) : No_GC_Verifier(verifygc){}
261 ~No_Safepoint_Verifier() {}
262 #endif
263 };
264
265 // A Pause_No_Safepoint_Verifier is used to temporarily pause the
266 // behavior of a No_Safepoint_Verifier object. If we are not in debug
267 // mode then there is nothing to do. If the No_Safepoint_Verifier
268 // object has an _activated value of false, then there is nothing to
269 // do for safepoint and allocation checking, but there may still be
270 // something to do for the underlying No_GC_Verifier object.
271
272 class Pause_No_Safepoint_Verifier : public Pause_No_GC_Verifier {
273 private:
274 No_Safepoint_Verifier * _nsv;
275
276 public:
277 #ifdef ASSERT
278 Pause_No_Safepoint_Verifier(No_Safepoint_Verifier * nsv)
279 : Pause_No_GC_Verifier(nsv) {
280
281 _nsv = nsv;
282 if (_nsv->_activated) {
283 _nsv->_thread->_allow_allocation_count--;
284 _nsv->_thread->_allow_safepoint_count--;
285 }
286 }
287
288 ~Pause_No_Safepoint_Verifier() {
289 if (_nsv->_activated) {
290 _nsv->_thread->_allow_allocation_count++;
291 _nsv->_thread->_allow_safepoint_count++;
292 }
293 }
294 #else
295 Pause_No_Safepoint_Verifier(No_Safepoint_Verifier * nsv)
296 : Pause_No_GC_Verifier(nsv) {}
297 ~Pause_No_Safepoint_Verifier() {}
298 #endif
299 };
300
301 // A SkipGCALot object is used to elide the usual effect of gc-a-lot
302 // over a section of execution by a thread. Currently, it's used only to
303 // prevent re-entrant calls to GC.
304 class SkipGCALot : public StackObj {
305 private:
306 bool _saved;
307 Thread* _t;
308
309 public:
310 #ifdef ASSERT
311 SkipGCALot(Thread* t) : _t(t) {
312 _saved = _t->skip_gcalot();
313 _t->set_skip_gcalot(true);
314 }
315
316 ~SkipGCALot() {
317 assert(_t->skip_gcalot(), "Save-restore protocol invariant");
318 _t->set_skip_gcalot(_saved);
319 }
320 #else
321 SkipGCALot(Thread* t) { }
322 ~SkipGCALot() { }
323 #endif
324 };
325
326 // JRT_LEAF currently can be called from either _thread_in_Java or
327 // _thread_in_native mode. In _thread_in_native, it is ok
328 // for another thread to trigger GC. The rest of the JRT_LEAF
329 // rules apply.
330 class JRT_Leaf_Verifier : public No_Safepoint_Verifier {
331 static bool should_verify_GC();
332 public:
333 #ifdef ASSERT
334 JRT_Leaf_Verifier();
335 ~JRT_Leaf_Verifier();
336 #else
337 JRT_Leaf_Verifier() {}
338 ~JRT_Leaf_Verifier() {}
339 #endif
340 };
341
342 // A No_Alloc_Verifier object can be placed in methods where one assumes that
343 // no allocation will occur. The destructor will verify this property
344 // unless the constructor is called with argument false (not activated).
345 //
346 // The check will only be done in debug mode and if activated.
347 // Note: this only makes sense at safepoints (otherwise, other threads may
348 // allocate concurrently.)
349
350 class No_Alloc_Verifier : public StackObj {
351 private:
352 bool _activated;
353
354 public:
355 #ifdef ASSERT
356 No_Alloc_Verifier(bool activated = true) {
357 _activated = activated;
358 if (_activated) Thread::current()->_allow_allocation_count++;
359 }
360
361 ~No_Alloc_Verifier() {
362 if (_activated) Thread::current()->_allow_allocation_count--;
363 }
364 #else
365 No_Alloc_Verifier(bool activated = true) {}
366 ~No_Alloc_Verifier() {}
367 #endif
368 };
369
370 #endif // SHARE_VM_MEMORY_GCLOCKER_HPP